Solid state lighting arrays are used for a number of lighting applications. For example, lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources in applications including architectural and/or accent lighting. A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs), which may include inorganic LED chips and/or organic LED chips (OLEDs). Typically, solid state light emitting devices generate light through the recombination of electronic carriers (electrons and holes) in a light emitting layer or region of a LED chip. LEDs have significantly longer lifetimes and typically have significantly greater luminous efficiency than conventional incandescent and fluorescent light sources; however, LEDs are narrow-band emitters, and it can be challenging to simultaneously provide good color rendering in combination with high luminous efficacy.
Aspects relating to the subject matter disclosed herein may be better understood with reference to the 1931 CIE (Commission International de l'Eclairage) Chromaticity Diagram, which is well-known and readily available to those of ordinary skill in the art. The 1931 CIE Chromaticity Diagram maps out the human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=A λ−5/(eB/T−1) where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the blackbody locus yield pleasing white light to a human observer. The 1931 CIE Diagram includes temperature listings along the blackbody locus (embodying a curved line emanating from the right corner). These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light that is on or near the blackbody locus can thus be described in terms of their color temperature.
LEDs typically receive a direct current (DC) input signal or a modulated square wave input signal so that a constant current flows through the LEDs when in an “on” state. A current value is typically set to provide high conversion efficiency. LED light sources with variable intensity may be controlled by changing duty factor of a modulated square wave input signal.
Conventional lighting systems for use in buildings are powered by an alternating current (AC) source; accordingly, a LED-based light source for use in buildings typically includes an AC-DC power converter. An AC-DC power converter often represents a significant fraction of the overall cost of a LED-based light source, and power losses inherent to such a power converter reduces overall efficiency of the light source. Additionally, AC-DC power converters are generally not as reliable as LEDs, and therefore can limit the operating lifetime of a LED light source.
To avoid disadvantages associated with use of AC-DC power converters, it has been proposed to operate a LED light source directly from an AC power source without AC-DC conversion. Multiple groups or sets of series-connected LEDs may be powered by different portions of an AC waveform. For instance, one group may be powered on when the amplitude of the AC waveform is positive, and another group may be powered on when the amplitude of the AC waveform is negative; however, this simple driving scheme typically suffers from flicker and reduced efficiency. To provide somewhat improved efficiency, a full-wave rectifier may be used; however, the resulting light source still has limited efficiency and may exhibit flicker.
Since LEDs emit light with narrow wavelength spectrum, it is often necessary to utilize LEDs having different peak wavelengths (e.g., different colors) in a single LED light source in order to generate light with desirably high color rendering characteristics. If multiple groups of LEDs including LEDs having different peak wavelengths are utilized in a light source lacking an AC-DC power converter, however, then it may be challenging to avoid perceptible variations in color of light (e.g., with respect to area) output by such a light source, particularly if multiple LEDs having different peak wavelengths are distributed over a large area. Whether or not LEDs have different peak wavelengths, another challenge with utilizing multiple groups of LEDs in a light source lacking an AC-DC power converter (particularly when multiple LEDs distributed over a large area) is avoiding perceptible variations in intensity of light (e.g., with respect to area) output by such a light source.
Still another challenge associated with utilizing multiple groups of LEDs in a light source lacking an AC-DC power converter is thermal management—including efficiently dissipating heat generated by LEDs without overheating individual LEDs (which would shorten LED lifetime) and without needlessly increasing heatsink area (which would increase cost and size of a light source).
Another challenge associated with solid state lighting apparatuses includes providing the ability to vary beam patterns while avoiding use of mechanical elements that would require periodic maintenance and/or would be subject to failure long before the service life of solid state light emitters. Still another challenge associated with solid state light apparatuses includes providing the ability to vary color temperature without unduly increasing cost or complexity of a lighting apparatus.
Accordingly, a need exists for improved solid state lighting apparatuses and/or improved methods including use of solid state lighting apparatuses that can be directly coupled to an AC voltage signal, without requiring use of an on-board switched mode power supply. Desirable solid state lighting apparatuses and methods would exhibit reduced flicker, reduced variation in color with respect to area, reduced variation in light intensity with respect to area, and/or improved thermal management.